Acids and Alkalis, 1 81 



other flowers, will (hew the fame liquor to be alkaline by 

 turning; green." 



Paper (tained with infufions, or with the juices of the pe- 

 tals of fuch flowers as have been juft mentioned, though an 

 excellent teft for alkalis, is not Co eafily affected by acids as 

 litmus, and, by keeping, lofes in a (hort time much of its 

 fenfibilitv. 



To obtain frefli made tefts from vegetables, few of which 

 are to be met with in winter, was a matter of confiderable 

 moment. Mr. Watt found the red cabbage (braflica rubra) 

 to fumifh the beft. In its frefh It ate it is more fenfible both 

 to acids and alkalis than even litmus, and more decifive from 

 "its being naturally blue; turning green with alkalis, and red 

 with acids. 



Mr. Watt extracts the colouring matter from the thin 

 parts of the leaves only. He minces them finall, and digeffs 

 them in water about the heat of 120° for a few hours. If 

 not to be immediately ufed, as it is apt to fpoil, Mr. Watt 

 recommends to fpread the minced leaves on paper, to dry 

 them in a gentle heat, and then to put them in botiles well 

 corked. To prepare the teft, infufe or digeft thefe dried 

 leaves in water acidulated with fulphuric acid till they give 

 out their colour; then (train the liquor through a cloth, and 

 add to it whiting or chalk, ftirring it frequently, until it be- 

 comes a true blue, neither inclining to green nor to purple. 

 When this point is gained, filter it immediately; other- 

 wife it will become greenifh by ftanding on the chalk, It 

 will depofit fome gypfum, and, by adding a little fpirit of 

 wine, will keep good for fome days. Too much fpirit de- 

 llroys the colour. If wanted to be kept longer, neutralife 

 it by means of a fixed alkali inftead of chalk, But as none 

 of thefe means will prcferve the liquor long without 'requir- 

 ing to be neutralifed afrefli juft before it is ufed ; and as the 

 fermentation which it undergoes, and perhaps the alkalis or 

 fpirit of wine mixed with it, feem to leffen its fenfibility; in 

 order to prcferve its virtues, while it is kept in a liquid ftate, 

 N 3 fome 



